Keithley 486 Instruction Manual page 83

SECTION 3
Front Panel Oaeration
In general, as C, becomes larger, the noise gain becomes
larger. An application of where C, is very high is leak-
age measurements of capacitors. In this case, Input
ENols, must include the effects of the voltage source (Es)
used to bias the capacitor (any noise in the source volt-
age will increase the input noise).
When measuring leakage currents on capacitors larger
than lO,OOOpF, stability and noise performance can be
maintained by adding a resistor in series with the capac-
itor under test. The value of this resistor should be
around 1MR. For large capacitor values (>lpF), the
value of the series limiting resistor can be made lower in
order to improve settling times; however, values below
1OkQ are not generally recommended.
The resistor is not critical in terms of tolerance or
stability Any carbon composition resistor will prove
adequate.
3.16.3 Triboelectric and Piezoelectric
Effects
Unwanted currents may be generated due to triboelec-
tric and piezoelectric effects, and will subsequently be
measured by the Model 486/487.
Triboelectric Effects
Triboelectric currents are generated by charges created
between a conductor and an insulator due to friction.
Here, electrons rub off the conductor and create a charge
imbalance that causes the current flow. A typical exam-
ple would be electrical currents generated by insulators
and conductors rubbing together in a coaxial cable (see
Figure 3-20).
Figure 3-20. Triboelechic Effect
I
"Low-noise" cable greatly reduces this effect. It typically
uses an inner insulator of polyethylene coated with
graphite underneath the outer shield. The graphite
provides lubrication and a conducting equipotential
3-34
cylinder to equalize charges and minimize charge
generated by frictional effects of cable movement.
However, even low-noise cable creates some noise when
subjected to vibration and expansion or contraction, so
all connections should be kept short, away from
temperature changes (which would create thermal
expansion forces), and preferably supported by taping
the cable to a non-vibrating surface such as a wall,
bench, or rigid pole.
Other solutions to movement and vibration problems
include:
l
Removal or mechanical decoupling of the source of
vibration. Motors, pumps and other electromechani-
cal devices are the usual sources.
l
Stabilization of the test hookup. Securely mount or
tie down electronic components, wires and cables.
Shielding should be sturdy.
Triboelectric effects can also occur in other insulators
and conductors which touch each other. Therefore, it is
important to minimize contact between insulators as
well as conductors in constructing test fixtures and con-
nections for low current and high impedance.
Piezoelectric Effects
Piezoelectric currents are generated when mechanical
stress is applied to certain insulating materials. The
effect occurs in ceramics and other crystalline materials,
as well as some plastics used for insulated terminals
and interconnecting hardware. An example of a
terminal with a piezoelectric insulator is shown in
Figure 3-21.
1 Figure 3-21.
Piezoelectric Effect
To minimize the current due to this effect, it is important
to remove mechanical stresses from the insulator and
use insulating materials which have minimal piezoelec-
tric effects, such as sapphire, Teflon@, or PVC.